PETROGRAPHY 



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anorthosites make up the country. The most massive variety consists 

 of large, bluish labradorite crystals, up to an inch and a half in length, 

 with almost no traces of dark silicates. The rock is nearly pure 

 feldspar, but it has been subjected to powerful dynamic disturbances 

 and now exhibits a mass of larger nucleal fragments, often of con- 

 siderable size, surrounded by line breccias of the same. The specific 

 gravity of the large pieces, free from garnets, lies between 2.65 and 

 2.71, proving them to be labradorite. The thin sections bring out 

 the crushed condition still more strongly. Even small brecciated 

 cracks penetrate the larger pieces. The comminuted feldspar is 

 more or less altered, and often presents a fibrous or scaly mass of 

 sericite. Other darker varieties from this western border of Moriah, 

 contain bisilicates and tend to assume gneissoid forms, from the 

 alignment of these, in continuous bands. The plagioclase is much 

 smaller than in the first mentioned variety. The dark silicates are 

 green monoclinic pyroxene, deep brown, almost opaque hornblende, 

 and less common hypersthene, in about this order of abundance. 

 There is also more or less titaniferous magnetite. The two 

 pyroxenes are evidently original minerals and much of the brown 

 hornblende is also, but there are cases, later described where it 

 forms one of the zones in the reactionary rims, which give the impres- 

 sion, that some of it may be secondary. Deep pink garnets are 

 universal and often associated in a most intimate way with the 

 pyroxene. The same cracks pierce both minerals, and though the 

 line of demarcation is sharp, it makes the observer suspect that the 

 garnet has resulted from the pyroxene. The reactionary rims of 

 garnet give some added ground for this suspicion. But garnet 

 often appears alone with no bisilicate near it, and in such cases it is 

 probably an original mineral, as garnet often occurs in this relation 

 in plutonic rocks. A microscopic drawing of a typical anorthosite 

 is given by the author in a paper in the Amer. Jour. Sci. for 

 Aug. 1892, p. Ill, Fig. 2. 



Generally outlying from the main exposures of anorthosites and 

 separated from them by intervening gneiss are the areas of rocks, 

 marked gabbro on the map. The gabbros exhibit massive, coarsely 

 banded and very thinly laminated forms, and in several places the 

 unbroken transition can be followed from one into the other. 



In general, the massive gabbro makes the impression of a dark 

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